A pacemaker is an electrical device that provides precisely timed electrical signals to stimulate the heart. A pacing lead or electrode is a flexible insulated conductor terminated by a rigid body having an exposed contact point or tip which conducts the signals from the pacemaker to an appropriate location in the heart. While sometimes the lead is sutured into the heart muscle, more often it is inserted into the right ventricle or right atrium by the so-called transvenous approach. The same type of lead is used to conduct signals produced by the heart to various sensing and monitoring instruments.
Usually such pacing and sensing leads have fixation means near the tip of the device to maintain the tip in electrical contact with heart tissue adjacent to the tip. Such means have included fine wire hooks mounted adjacent to the tip which spring out into adjacent tissue or trabeculations within the heart. The hooks do achieve fixation. However, when it becomes necessary for one reason or another, to remove the lead, they are difficult to dislodge without causing at least some damage to heart tissue.
To avoid that problem, in lieu of hooks, more recent leads and electrodes of this general type are formed with flexible resilient plastic or silicone rubber projections which engage the trabeculations and cooperate with tissue ingrowth to maintain the lead tip in intimate electrical contact with stimulatable (viable) heart tissue. For example, U.S. Pat. No. 3,719,190 discloses a lead with a cone or parachute of resilient insulating material which extends out behind the lead tip. The cone is collapsible so that it can be passed through an introducing device or the vein itself when the electrode is being inserted. Also being resilient, it resumes its original shape upon leaving the introducing device or entering larger diameter veins so that after the lead tip is properly positioned in one of the heart cavities, the cone edge engages adjacent trabeculations and usually holds the lead in place until tissue ingrowth firmly anchors the lead. This type of lead can be withdrawn from the patient if that becomes necessary by pulling on the insulated conductor lead. This usually causes the parachute to invert and release from the tissue ingrowth and present a tapered face in the withdrawal direction so that it can be pulled back through the vein of introduction and/or the introducing device.
This prior lead is disadvantaged in that the cone or parachute, when collapsed, still has a relatively large cross section so that a relatively large introducing device or vein is required in order to insert that lead into the patient. This is undesirable because it requires a correspondingly large incision in the patent. Also, when the lead tip is positioned within the heart, the circular edge of the parachute does not always engage or grab the adjacent heart tissue or trabeculations firmly enough to fix the position of the tip until there has been sufficient tissue ingrowth to anchor the lead. Furthermore, even though that lead is designed to be removable, in practice, the parachute exhibits sufficient resistance to inversion as to make it relatively difficult to withdraw the lead back out through the introducing device or catheter.
U.S. Pat. Nos. 3,902,501 and 3,939,843 disclose endocardial leads with three or four outwardly-rearwardly extending pliant uniformly cylindrical tines for holding the leads in place within the heart. While these tines provide better retention of the lead then the aforesaid parachute, they also have certain drawbacks. More particularly, the tines, which vary in length from 2 to 5 mm, have very small cross sections, e.g. 1 to 2 mm, so that there is very little material at the roots of the tines where they join the lead body. In some cases, a pulling force of as little as 11/2 pounds is enough to separate a tine or a portion thereof from the remainder of the lead. A pulling force of this order can easily be experienced when it becomes necessary to reposition the lead by pulling or manipulating the lead's insulated conductor after the lead has become dislodged due to heart wall movements.
Another occasion for such tine separation is when the lead is being inserted into the heart across the tricuspid valve. If the tines are of the type which are relatively long and stiff, they can become caught in the valve and such separation may occur during efforts to free the lead, not infrequently causing inversions and/or damage to valve leaflets in the process. Tine separation can also occur when the lead is being retracted through the introducing device. The introducing device is a special hollow catheter that is inserted into the patient's vein, commonly the subclavian vein, in order to facilitate the insertion of and to guide the lead and to eliminate the need for the more time consuming venous cut down. The distal or inner end of this introducing device has a small diameter and a thin wall so that it presents a relatively stiff and sharp edge. When a tined lead is being inserted into a patient, its tines are pressed against the lead body by the wall of the introducing device. However, as soon as the tines pass through the distal or inner end of the introducing device, they spring out to their normal extended positions. Sometimes, however, when threading the lead along the venous course to, or during repositioning in the heart, it becomes necessary to pull the lead body back into the introducing device. In the process, the fine rearwardly angled tines are drawn against the stiff sharp edge of the introducing device causing cuts in the tines. In extreme cases, the cutting can be sufficiently severe to greatly weaken the tines or even to separate them from the remainder of the lead. This same cutting problem also arises when the lead is withdrawn from a patient after it is no longer needed. Suffice it to say, the presence of a loose tine or segment thereof in a patient's venous system may constitute a life-threatening hazard to the patient.
A tined lead is disadvantaged also in that, like the parachute electrode, it requires an introducing device with a relatively large lumen. In other words, even when the tines are folded back against the lead body, the overall body cross section is still relatively large. For example, a typical lead with either long or short tines requires a No. 9 introducing device as measured on the French Catheter Scale.
Also the tines on such a lead, being tiny cylinders that project out from the lead body have no preferred direction of deflection or bending. Thus, when the lead is being inserted through an introducing device or vein, the tines may not lie symmetrically about the axis of the lead body. In other words, they may extend in different directions or be wrapped to a greater or lesser degree one way or another about that body. When the tines are relatively long, one tine may even overlap another tine thereby increasing the effective cross section of the lead. Resultantly, the lead may not feed through the introducing device and vein particularly smoothly and uniformly thereby making it more difficult for the surgeon to insert the lead into a patient. Similarly, when the lead has to be withdrawn, the tines do not necessarily simply invert. Again, they may extend and wrap in different directions about the lead body making it more difficult if not impossible to withdraw the lead through the patient's vein. Such random deflection and wrapping also increases localized stresses on the individual tines increasing the possibility of a tine or a portion thereof breaking away from the body of the lead.